EAS systems of the type described above, are, for example, disclosed and claimed in U.S. Pat. No. 3,665,449 (Elder and Wright). As set forth at Col. 5, lines 10 to 39 therein, the high-coercive force section of a marker may be magnetized by placing it in the field of a large permanent magnet of sufficient intensity, and gradually removing the field, such as by withdrawing the marker therefrom.
While such a technique may be useful in many areas and with the markers affixed to a wide variety of articles, the magnetic fields associated therewith have been found to unacceptably interfere with magnetic states associated with certain articles. For example, the compact size and popularity of prerecorded magnetic audio and video cassettes make such articles frequent targets for shoplifters, and hence likely articles with which anti-theft markers would be used. At the same time however, such affixed markers would be desirably desensitized upon purchase, and it has been found that certain prior art desensitizer apparatus such as described above may unacceptably affect signals prerecorded on magnetic tapes within the cassettes.
To avoid such deleterious effects on prerecorded magnetically sensitive articles, it is also known to provide apparatus in which a steady-state field is produced which rapidly decreases in intensity only a short, controlled distance from the apparatus. Thus, such an apparatus, while being capable of magnetizing high-coercive force sections of a marker brought close thereto, would be incapable of interfering with the magnetic signals recorded on tapes within a cassette to which the marker is affixed. See U.S. Pat. No. 4,499,444 (Heltemes and Montean). The apparatus there described comprises a permanent magnet assembly which includes at least one section of a permanent magnet ferromagnetic material having two substantially opposed major surfaces and a pair of pole pieces each of which is proximate to and extends over a major portion of the major surfaces and terminates proximate to the other pole piece, leaving a gap therebetween of substantially constant width extending along the length of the permanent magnet material. The permanent magnet material is substantially uniformly magnetized to present one magnetic polarity at one of the major surfaces and the opposite polarity on the other major surface. The pole pieces in turn concentrate external magnetic lines of flux resulting from the magnetized material near the gap. The resultant external magnetic field decreases rapidly with increasing distance from the gap, and enables a marker to be moved relative to the gap to magnetize the section of said high coercive force material within the marker while not altering magnetic states such as may exist within an article to which the marker is secured.
An apparatus such as described in the aforementioned U.S. Pat. No. 444 has generally been found to be satisfactory so long as it is used with markers of a single type, and whose magnetizable components all have a coercive force within a given range, such that the field intensity at the working surface of the apparatus is controlled to appropriately magnetize those components while not adversely affecting magnetically sensitive articles. Conversely, it has been found that when the apparatus is used with markers nominally of the same type, but in which the value of the coercive force varies over a relatively wide range of allowed values, certain conditions may cause unsatisfactory results.
For example, to prevent adverse effects on magnetically sensitive articles with which the markers are desirably used, the field intensity at some distance from the working surface of the apparatus at which such magnetically sensitive articles are to be located, must be below certain design limits. However, a practical apparatus desirably has an effective operable range extending a short distance above the surface within which all allowed materials must become magnetized. Some materials having coercive forces near the highest allowed value and positioned near the outer edge of the allowed range, i.e., in the weakest fields, may not become sufficiently magnetized. And, since there is typically a reverse directed back field, which is particularly strong near the surface of the apparatus, such back fields may be sufficient to reduce the magnetization state in materials near the surface and having coercive forces near the lowest allowed value. Such reduced magnetization levels could, in turn, inadequately bias the low coercive, high permeability material of the marker, such that the response of the marker would be inadequately altered. Such effects are further compounded and totally unacceptable results may occur, if markers of significantly different types, each having magnetizable materials having coercive forces in significantly different ranges are used with the same apparatus.